Field of the Invention
The present invention relates to a method for producing a catalytic converter being formed of a honeycomb body with flow channels having catalytically active surfaces.
It is known in the state of the art how to apply catalytically active materials to the surfaces of the flow channels of a honeycomb body. The honeycomb body basically is formed of a metallic or ceramic support material. A porous intermediate layer (washcoat) that enlarges the active surface is typically applied to the support material. In that way, the geometrical surface of the flow channels is multiplied by several orders of magnitude. A catalytically active material is then applied on the resulting active surface. Combustion exhaust gases, e.g., from an automotive engine, are led through the flow channels and particular components of the combustion exhaust gas are converted by the catalytic action in the region of the coated surfaces, typically exothermally, so that a catalytically treated combustion exhaust gas emerges from the outlet of the flow channels.
However, the catalytic action of such a catalytic converter does not last without limit, even though the catalytically active materials themselves are not converted. That is due to various mechanisms. First of all, the combustion exhaust gas typically contains a fraction of particles (albeit slight) which can settle into the flow channels and thus reduce the active surface of the catalytic converter. In other words, the cavities of the porous intermediate layer become filled with foreign material. The free cross section of the flow channels may even remain unaffected. Often, the combustion exhaust gas also contains slight amounts of substances which inhibit the catalytic action of the catalyst materials. Deposits of such substances in the honeycomb body of the catalytic converter can result in total disabling (poisoning). Furthermore, the catalytic converter is exposed to continual mechanical stresses, namely vibrations (particularly when used in automobiles), as well as thermal expansion and contraction (from constantly changing operating conditions, especially stop and go operation). That can result in pieces of the porous intermediate layer becoming detached from the support body, which again loses part of the active surface. Finally, the high temperatures occurring in the exothermal conversion of a component of the combustion exhaust gas lead to a glazing or vitrification of the porous intermediate layer. Extremely high temperatures, for example, can be caused by incomplete combustion in the engine, so that some of the combustion occurs catalytically in the catalytic converter. The gradual glazing of the porous intermediate layer means that the combustion exhaust can no longer reach the deeper lying voids in the porous intermediate layer. The active surfaces of those voids are then no longer available for the catalytic conversion and thus diminish the total active surface.
However, the individual components of a catalytic converter have different service lives. While the catalytic material is basically unconsumed, it is gradually lost for the effectiveness of the catalytic converter in the course of its operating life, for the above-given reasons. Thus, the effectiveness of the catalytic converter is primarily determined by the integrity of the porous intermediate layer. In particular, at the time when the catalytic converter has to be replaced in the automobile, by reason of legal requirements, for example, the support body is still intact. In particular, metal support bodies can have a service life that is often twice as long as that of the porous intermediate layer.
The still young technology of catalysts for motor vehicles has established thus far that the service life of the catalytic converter is considerably less than the service life of the motor vehicle in which the catalyst is installed. The service life of the catalyst until recently corresponded to only around 50,000 km of vehicle mileage. It is common practice to subject a catalytic converter at the end of its service life to a process by which the noble metals can be recovered. Examples of such processes will be found in German Published, Non-Prosecuted Patent Applications DE 41 05 224 A1 and DE 41 22 717 A1. The proposed methods concentrate on recovering the noble metal components, since they represent a not inconsiderable value. However, the other metal components can also be subjected to recycling.